help me calculate the slew rate

[wwwww12345]

Hi all,

I donot know how the slew rate in this paper is calculated . Does anyone know , please tell me ,thank you!

 

[tshah]

Slew rate is basically a degradation effects on the high frequency response of active amplifier due to compensating capacitor. Slew rate is due to the maximum supplied current available for charging up the compensating capacitor. It can be found using a formula

slewrate = I(max)/C

So, if you have a total output current of 1 ma and if the compensating capacitor is 1000 pF, the slew rate is

SR = 1ma/1000 pF = 1 V/us

Also, you want to make sure that slew rate > 2*pi*f*v(peak)
V(peak) is the largest possible amplitude of the input signal to avoid distortion due to slewing.

So, if your slew rate is 0.5 and frequency is 20000 Hz, then

v(peak) = 0.5 / (2*pi*20000) = 3.98 volts.

These are general guidelines from me.

You can use it for your calculations.

Thanks

 

[wwwww12345]

Hi tshah，

Thank for your reply.
Can you tell me why SR>2*pi*f*V(peak)?

Let us turn to this problem. If we look the schematic as a two stage opamp, the first stage SR is Ibias/C, while the second is Itail/Cout. How can he calculate the formula 1?
Can someone tell me ? thanks !

 

[Alex_IC]

The SR of the first diff amp stage is Ibias/C and it is gained by Gm of the second stage which is Itail/Von. It is like output voltage SR of an opamp can be gained with output stage, if the output stage has gain more than 1.

The SR of a sine wave is 2*pi*f*V(peak), so you need SR of the amplifier more than SR>2*pi*f*V(peak) in order to avoid large distortion (sine wave would be triangular if SR not big enough).

 

[pkbhardwaj]

just calculate the final output and input relationship

 

[wwwww12345]

The SR of the first diff amp stage is Ibias/C and it is gained by Gm of the second stage which is Itail/Von. It is like output voltage SR of an opamp can be gained with output stage, if the output stage has gain more than 1.

The SR of a sine wave is 2*pi*f*V(peak), so you need SR of the amplifier more than SR>2*pi*f*V(peak) in order to avoid large distortion (sine wave would be triangular if SR not big enough).

Thank you! Alex !